This invention relates to methods for testing linear differential line drivers, and more particularly relates to testing linear output differential voltage parameters.
A widely used technology for interconnecting devices on a network is EIA-485, also known as RS-485. The names for this technology reflect the fact that it is compliant with the TIA/EIA-485-A standard (hereinafter, xe2x80x9cthe EIA-485 standardxe2x80x9d), described in xe2x80x9cElectrical Characteristics of Generators and Receivers for use in Balanced Digital Multipoint Systems,xe2x80x9d March 1998, available from the Telecommunications Industry Association. The EIA-485 standard specifies the characteristics of the generators, or transmitters, and receivers used in a digital multipoint system. A digital multipoint system is one in which two or more transmitters and one or more receivers are connected by way of a single bus. The standard describes such parameters as unit loads, output drive, short circuit current and common mode voltage. The standard does not specify other qualities of the system such as signal quality, protocol, timing pin assignments, power supply voltages or operating temperature range. Basically, according to the standard, a driver must be able to source at least 1.5 volts differentially into 60 ohms (two 120 ohm terminators in parallel along with 32 unit loads) under a common mode voltage range of xe2x88x927 to +12 direct current volts, Vdc.
An EIA-485 bus is a balanced transmission line that has two wires, other than ground, typically twisted pair, identified as A and B. The transmission line is considered balanced, because the signal on one wire is ideally the exact opposite of the signal on the other wire, i.e., a driver generates complementary voltages on A and B. Thus, if one wire is transmitting a High the other wire will be transmitting a Low, and vice versa.
In EIA-485 systems, drivers are typically implemented using bi-directional transceiver packages, such as the SN65176B and SN75176B Differential Bus Transceiver integrated circuits manufactured by Texas Instruments Incorporated. It is necessary for the manufacturer of such transceiver packages to test the Output Differential Voltage, VOD, on the drivers. Such tests have been typically performed using resistive components on specialized test boards made up to fit on the tester machines used in the fab for testing packages that come off the line. However, passing components through these test boards limits manufacturing line throughput, as the test boards take time to install. In addition, such test boards are expensive to fabricate, and are usually specific to only one product or class of products, adding to the overall cost of manufacture. It would therefore be desirable to have a method for testing linear differential devices, such as EIA-485 transceiver ICs, that is less costly and time consuming.
In accordance with the present invention there is provided a method for measuring a test differential voltage across a first output and a second output of a transmitter integrated circuit device, the test differential voltage corresponding to a voltage across the first output and second output appearing while the device is providing an output while being subjected to a voltages applied across a resistor network connected to the differential outputs, the resistor network comprising a first resistor having a value of Ra connected between the first output and a first voltage, a second resistor having a value of Rb connected between the second output and a second voltage, and a third resistor having a value of Rc connected between the first output and the second output. In the method, a first current is applied to the first output, the first current being of a magnitude determined to correspond to a magnitude of current that would appear at the first output while the device were providing an output while being subjected to said predetermined voltages applied across said resistor network. A second current is applied to the first output, the second current being of a magnitude determined to correspond to a magnitude of current that would appear at the second output while the device were providing an output while being subjected to said predetermined voltages applied across said resistor network. Finally, a test differential output voltage is measured across the first output and the second output.
These and other features of the invention will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings.